A large number of transition metal complexes are used as catalysts for organic synthesis reactions. Particularly, being stable and easy to handle, noble metal complexes have been used as catalysts in various synthesis studies in spite of their expensiveness, and many reports have been published stating that their use has rendered possible organic synthesis reactions which were not possible by conventional means. There are various types of optically active ligands to be used in such asymmetric catalysis reactions, including 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (to be referred to as "BINAP" hereinafter) ligand which is one of the catalysts having most excellent asymmetry recognizing ability. For example, Ru complexes having BINAP as a ligand show markedly excellent catalytic activity and enantio-selectivity for the asymmetric hydrogenation of olefins and ketones.
However, they are not so effective for the asymmetric hydrogenation reaction of relatively simple olefins having no heterofunctional groups in the substrate, and very little actually is known about catalyst systems which are effective for the asymmetric hydrogenation of such olefins.
Recently, an asymmetric ligand 2-(diphenylphosphino)-1,1'-binaphthalen-2'-yl) (1.1'-binaphthalen-2,2'-yl)phosphite (to be referred to as "BINAPHOS" hereinafter) having a binaphthyl nucleus but no C2 chirality has been reported by Sakai et al. (J. Am. Chem. Soc., vol.115, p.7033, 1993), which showed excellent performance in the asymmetric hydroformylation reaction of olefins.
However, such a compound having an optically pure binaphthyl nucleus has difficulty in effecting modification of functional groups in comparison with other compounds derived from optically pure tartaric acid or amino acids, and reports on the synthesis of its derivatives and asymmetric ligands are extremely scarce.
As described above, great effort has been directed toward the development of an asymmetric diphosphine ligand as a catalyst for asymmetric synthesis reactions, which shows a certain substrate selectivity, conversion ratio, catalytic activity, optical purity and the like that are different from those of the prior art BINAP derivatives. The present invention contemplates satisfying such demands.
It is accordingly an object of the present invention to provide a novel process for the asymmetric hydrogenation of optically active .alpha.-alkylstyrene derivatives, in which such an asymmetric diphosphine and a transition metal are used.